Zeolites are crystalline aluminosilicate microporous compositions which are formed from corner sharing AlO4/2− and SiO4/2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in a variety industrial and environmental applications. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on the outside surfaces of the zeolite as well as on internal surfaces within the pores of the zeolite.
Over the years, new chemistry has been designed to adapt the hydrothermal synthesis techniques to produce microporous materials of other compositions, e.g., non-zeolitic compositions. In 1982, Wilson et al. first reported aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of these same properties of zeolites, although they are silica free, composed of AlO4/2− and PO4/2+ tetrahedra (See U.S. Pat. No. 4,310,440). The AlPOs are formally neutral, they do not possess framework charge. Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for PO4/2+ tetrahedra to produce the SAPO molecular sieves (See U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [M2+O4/2]2− tetrahedra for AlO4/2− tetrahedra, which yield the MeAPO molecular sieves (see U.S. Pat. No. 4,567,029). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the simultaneous introduction of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (See U.S. Pat. No. 4,973,785 and U.S. Ser. No. 10/370,257). These groundbreaking patents opened new territory upon which technologists have built ever since, introducing new chemistry that has increased the diversity of compositions and structures known among the members. One such advance was the synthesis of SAPO-56, which is described in U.S. Pat. No. 5,370,851. SAPO-56 has the AFX topology (See Database of Zeolite Structures; http://www.iza-structure.org/databases/) and is prepared with the N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD) structure directing agent (SDA). A detailed study of this material found that the SAPO-56 product derived from the TMHD-based preparation contained extraframework Si, yielding a formulation of TMHD3.26(Al23.5P19.5Si4.9)O96 (See Microporous and Mesoporous Materials, 1999, 28, 125-137). The Si incorporation level into the aluminophosphate-based structure is given by 5 Si4+/(23.5 Al3++19.5 P5++5 Si4+)=10.4% Si substitution into the framework. The negative framework charge density (FWCD) in units of charge/T-atom of this SAPO-56, given ([AlO4/2]−+[PO4/2]+)/([Al]+[P]+[Si])=(−23.5+19.5)/(23.5+19.5+5)=−0.083/T-atom.
Aluminosilicate compositions having the AFX topology have been prepared by Zones (See U.S. Pat. Nos. 4,508,837 and 5,194,235) using diquats based on two quinuclidine or two DABCO molecules linked by a 3-5 carbon chain. When another diquat based on linking two N-methylpyrrolidinium molecules via a butane moiety gave the aluminosilicates TNU-9 (See F. Gramm et al., Nature (London), 2006, 444, 79) and TNU-10 (S. B. Hong et al., J. Am. Chem. Soc., 2004, 126, 5817), Maple and Williams used similar N-methylpyrrolidinium-based SDAs with linkages varying from 3-5 carbons to successfully prepare MgAlPO compositions with the AFX topology (Dalton Trans., 2007, 4175). Their attempts to make a pure AFX topology material in a SAPO composition using the same structure directing agents (SDAs) were unsuccessful. Recently Xie et al. brought this chemistry back to aluminosilicates, employing the SDAs 1,4-bis(methylpyrrolidinium)butane, 1,6-bis(N-methylpyrrolidinium)hexane and 1,6-bis(trimethylammonium)hexane (hexamethonium) to make AFX topology aluminosilicates (See U.S. Ser. No. 10/053,368). An aminothermal approach to make SAPO AFX compositions by Wang et al. (See CrystEngComm, 2016, 18, 1000). Using high concentrations of the volatile SDAs triethylamine (TEA) and trimethylamine (TMA) along with hydrofluoric acid, a SAPO-AFX with the composition (TEA)4.4(TMA)2.2(H2O)8[Al20.7P16.3Si11.0O96] was prepared. The highly volatile TEA and TMA along with dangerous HF are problematic reagents to use on large scale as is the low water levels, which hamper the homogenization required on large scale. The composition exhibits high Si incorporation (11/(20.7+16.3+11)=22.9% of framework atoms), but the FWCD=(−20.7+16.3)/(20.7+16.3+11), which is −0.092/T-atom. More recently, Casci et al. in U.S. Pat. No. 10,029,239 also uses the TMA SDA in combination with the DABCO-based SDAs utilized in the preparation of aluminosilicate AFX compositions mentioned above (See U.S. Pat. No. 5,194,235) to make a SAPO composition with the AFX topology. No composition is disclosed, but the large size of the linked DABCO-based diquat SDAs would suggest a lower charge density than observed in the other AFX compositions disclosed above.
In contrast to the work disclosed above, applicants have synthesized high FWCD SAPO and MeAPSO (Me=Zn2+, Co2+, Mn2+ and Mg2+) compositions with the AFX topology, designated SAPO-80. The use of the more compact α,α′-bis(dimethylethylammonium)-p-xylene diquat SDA enables applicants to achieve AFX compositions with FWCD>−0.10/T-atom. The higher FWCD can enhance ion exchange capacity and the acid site density over that found in other SAPO-based AFX compositions. The SAPO-80 compositions are stable to SDA removal by calcination and may be used as adsorbents or catalysts.